Rapid Electron Transfer within the III-IV Supercomplex in Corynebacterium glutamicum

Area of Science:

• Biochemistry
• Molecular Biology
• Microbiology

Background:

• * *Corynebacterium glutamicum* possesses a unique respiratory supercomplex comprising Complex III and Complex IV.
• * Complex III features an unusual di-heme cytochrome c1, co-purifying with Complex IV.
• * Understanding electron transfer within this supercomplex is key to deciphering bacterial respiration.

Purpose of the Study:

• * To investigate the kinetics of electron transfer within the *C. glutamicum* Complex III-IV supercomplex.
• * To elucidate the role of the di-heme cytochrome c1 in electron transfer pathways.
• * To compare electron transfer in the supercomplex versus Complex IV alone.

Main Methods:

• * Genetic removal of Complex III (cytochrome bc1) to isolate Complex IV (cytochrome aa3).
• * Kinetic analysis of electron transfer reactions using spectrophotometry.
• * Investigating electron transfer from reduced cytochrome aa3 to oxygen.
• * Studying electron transfer from externally added cytochrome c to Complex IV.

Main Results:

• * The reaction of reduced cytochrome aa3 with O2 followed known pathways for A-type oxidases, with no observed pH dependence despite proton uptake.
• * In the supercomplex, electron transfer from cytochrome c hemes to CuA occurred within 0.1-1 ms.
• * Oxidation of b-hemes showed a rate-limiting time constant of 6.5 ms, impacting overall quinol oxidation/O2 reduction.
• * Removal of Complex III accelerated electron transfer from external cytochrome c to Complex IV, suggesting proximity of a c-heme to CuA.

Conclusions:

• * Isolation of the Complex III-IV supercomplex enabled detailed kinetic studies of electron transfer.
• * The di-heme cytochrome c1 acts as a critical intermediary in electron transfer from b-hemes to the heme a/heme a3-CuB catalytic site.
• * Findings highlight the functional integration of Complex III and IV in bacterial respiration.